<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<h1>0</h1>
<h2>0</h2>

<script>
    let count = 0;
    let num = 0;
    //开启定时器 每隔一秒调用一次方法
    //方法作为参数进行传递时不写()
    setInterval(f,1000);
    function f(){
        count++;
        let h = document.querySelector("h1");
        h.innerText = count;
    }

    let timer = setInterval(function (){
        num++
        let h = document.querySelector("h2");
        h.innerText = num;
        if(num == 50){
            //停止定时器
            clearInterval(timer)
        }
    },100);

    //只执行一次的定时器

    setTimeout(function (){
        alert("时间到！")
    },3000);
</script>
</body>
</html>